The photo of the ring probably does not do justice to this lovely creation. But from looking at your photo, at looks like a CZ (not a real diamond) or possibly a crystal. Please give us more info. Thanks.
Do you have Tiffany papers for the ring showing the carats of the diamond? You say that there is a marking on the gold indicating that it is 18K? Time is running out. Nobody is being rude. Just inquiring about important data so that we can make a valid decision on our bidding. Being too busy to give facts may cost you.
from looking at picture, the "diamond" appears to be a .75 Ct. or even t.00 Ct. You should have a jeweler check it out for size and whether a real diamond or CZ. If real genuine diamond (not CZ) then this ring is worth a lot.
18Kts indicates gold in ring. Carats of diamond/CZdiamond would be: example .20CT. or .66Ct. or .50Ct. Just for your info. Great looking ring. Do you know the size of the diamond/CZ diamond?